Amplifier linearization using RF feedback and feedforwardtechniques

The performance of feedback as a distortion reduction technique is highly dependent on the integrity of the feedback path. Any error or noise generated in this path is directly reflected into the output of the amplifier. Linearized RF power amplifiers (PAs) using Cartesian feedback require a demodulator in the feedback loop, and this is a potential source of linear errors, nonlinear errors, and noise. RF feedback with Cartesian compensation is proposed as a technique for overcoming some of these problems. The scheme is most suited to systems requiring an RF input. In addition, the RF nature of the input, feedback, and error signals enables the addition of a feedforward loop to further improve the linearization capability while still maintaining good efficiency. Design equations and simulation results are given for such a system. Disadvantages include the limited bandwidth (estimated at 1 MHz) and the need for additional circuits to generate the RF input signal when included in an integrated transmitter     

Amplifier linearization using compact microstrip resonant cell-theory and experiment

This paper presents a novel technique for reducing the intermodulation distortions (IMDs) in power amplifiers. In this method, both second- and third-harmonic components generated by the transistor are reflected back simultaneously by the compact microstrip resonant cell (CMRC) at the input port with proper phases to mix with the fundamental signal for the reduction of IMDs. A rigorous mathematical analysis on the effectiveness of multiharmonic reflections has been formulated and derived using the Volterra series. Moreover, the delay mismatch factor of the proposed method is analytically studied and the result shows that a better tolerance to the delay error can be achieved by using CMRC circuitry. Standard two-tone test measurements reveal 32- and 22-dB reductions for the third-order IMD and fifth-order IMD, respectively, without affecting the fundamental signal at 2.45 GHz. Meanwhile, the proposed approach gives a peak power added efficiency of 53% with 11.5 dB transducer gain and 15 dBm output power for a single-stage SiGe bipolar junction transistor. The adjacent channel power ratio (ACPR) is -55dBc for a data rate of 384-kb/s quadrature phase shift keyed modulated signal with 2-MHz spanning bandwidth, and this ACPR is maintained for a broad range of output power level.     

Amplifier linearization using a digital predistorter with fastadaptation and low memory requirements

A method is described for linearizing a power amplifier by predistorting its input. It is particularly well suited to baseband implementation with digital signal processor hardware. In comparison with the most powerful previously published predistorter, it requires four orders of magnitude less memory, reduces convergence time by over three orders of magnitude, eliminates reconvergence time following a channel switch, and eliminates the need for a phase shifter in the feedback path. The predistorter structure is described. Its ability to suppress intermodulation products using only a small table is demonstrated. The effect of predistorter nonidealities (especially limited table size) on the power amplifier's output are analyzed. A fast adaptation algorithm is introduced     

Comments on "Counterexample to harmonic linearization"

A recently proposed example for invalidating harmonic linearization method is shown to be incorrectly analyzed. The two time-scaling technique has been shown to yield the correct solution.     

Theoretical and experimental study of amplifier linearization basedon harmonic and baseband signal injection technique

This paper presents a novel linearization scheme for RF amplifiers based on simultaneous harmonic and baseband signal injection. In this method, second-order frequency components generated by predistortion circuits are fed to the input of the main amplifier to mix with the fundamental signal for third-order intermodulation distortion (IMD) cancellation. A general and rigorous analytical formulation of baseband, harmonic, and the proposed injection techniques is presented, and from these derived expressions, the optimum conditions for IMD suppression are developed. The result also reveals the practical limitation of the proposed method subject to gain and phase error associated with the RF and baseband circuitry. For comparison purposes, an amplifying system is constructed for the experimental investigation of second-order signal injection approach. Both two-tone and digitally modulated waveforms are employed in these measurements     

A composite varactor for simultaneous high power and high efficiency harmonic generation

The proposal of connecting several varactor wafers in series to obtain higher breakdown voltages without loss of cutoff frequency is explored theoretically and experimentally. Calculations indicate that for a common wafer and junction geometry, the cutoff frequency of series-stacked varactors will vary approximately as VB^-1/2instead of the more rapid VB^-1or VB^-3/2variation characteristic of a single wafer. The virtue of this scheme has been demonstrated in a 4-to-12 Gc/s multiplier using varactors consisting of 1, 2, 3, and 4 series-connected, epitaxial GaAs, surface barrier wafers. As anticipated, the varactor breakdown voltages increased directly with the number of wafers employed and the power capability as the square of that number, while efficiency decreased only slightly. A maximum power output of 1.2 watts at 12 Gc/s was obtained with 62 percent efficiency in a tripler circuit.     

Efficiency-optimized direct torque control of synchronousreluctance motor using feedback linearization

In this paper, a nonlinear controller capable of high dynamic torque regulation and efficiency optimization of the synchronous reluctance motor (SynRM) using input-output feedback linearization is proposed. The cross-coupling effects in the SynRM model and the torque nonlinearity due to the iron losses in torque-speed characteristics of the SynRM are discussed. The criterion for the efficiency optimization is also introduced and investigated. Since the proposed nonlinear controller directly regulates the torque by selecting the product of d- and q-axes torque currents as one of the output variables, the nonlinear and cross-coupling aspects between the d-and q-axes torque currents and the terminal currents can he eliminated. Hence, the linear torque-speed characteristic can be achieved. In addition, by controlling the power loss-minimizing criterion directly, the proposed controller can optimize the efficiency of the SynRM without deteriorating the dynamics performance     

Calculation of timing and amplitude jitter in dispersion-managedoptical fiber communications using linearization

An approach based on linearization that allows us to calculate the timing and amplitude jitter for arbitrary pulse shapes in dispersion-managed fibers is developed. We apply this approach to calculate the jitter for dispersion-managed soliton, return-to-zero (RZ), and nonreturn-to-zero (NRZ) transmission formats. We then estimate the bit error rates. The approach described here yields more precise results than Monte Carlo simulations at a fraction of the computational cost     

A dual SQUID linearization concept using a phase modulation scheme

A critical control system evaluation is presented of basic flux-locked loop systems. The development of a new superconducting quantum interference device (SQUID) linearization method is then described, where no magnetic flux feedback is necessary to cancel the applied flux. It is shown that a dual SQUID configuration will be able to produce a true phase modulation system that is easily demodulated with a phase-locked loop. The theoretical performance of the proposed configuration is verified by simulations, and the performance and limitations are discussed in detail. It is shown that small dc correction voltages at the output of the SQUID's significantly decrease output noise, as is the case with an increase in SQUID dc bias currents. An optional feedback system is also described for optimal performance of the dual SQUID configuration     

2-D harmonic retrieval using higher-order statistics

We consider the problem of estimating the harmonics of a noisy 2-D signal. The observed data is modeled as a 2-D sinusoidal signal, with either random or deterministic phases, plus additive Gaussian noise ofunknown covariance. Our method utilizes recently defined higher-order statistics, referred to as mixed-cumulants, which permit a formulation that is applicable to both the random and deterministic case. In particular, we first estimate the frequencies in each dimension using an overdetermined Yule-Walker type approach. Then, the 1-D frequencies are paired using a matching criterion. To support our theory, we examine the performance of the proposed method via simulations.     

Oxide interface studies using second harmonic generation

We report experiments using a non-invasive second harmonic generation (SHG) technique to characterize buried Si/SiO₂ interfaces and also SIMOX thin film silicon-on-insulator (SOI) wafers. The measurements demonstrate that the SHG response can provide an indication of the quality of the buried oxide interfaces, by providing information on surface roughness, strain, defects, and metallic contamination. The potential application of SHG for comprehensive buried interface characterization and as a non-contact metrology tool for process control is described.     

Surface-constrained volumetric brain registration using harmonic mappings

In order to compare anatomical and functional brain imaging data across subjects, the images must first be registered to a common coordinate system in which anatomical features are aligned. Intensity-based volume registration methods can align subcortical structures well, but the variability in sulcal folding patterns typically results in misalignment of the cortical surface. Conversely, surface-based registration using sulcal features can produce excellent cortical alignment but the mapping between brains is restricted to the cortical surface. Here we describe a method for volumetric registration that also produces an accurate one-to-one point correspondence between cortical surfaces. This is achieved by first parameterizing and aligning the cortical surfaces using sulcal landmarks. We then use a constrained harmonic mapping to extend this surface correspondence to the entire cortical volume. Finally, this mapping is refined using an intensity-based warp. We demonstrate the utility of the method by applying it to T1-weighted magnetic resonance images (MRIs). We evaluate the performance of our proposed method relative to existing methods that use only intensity information; for this comparison we compute the intersubject alignment of expert-labeled subcortical structures after registration.     

Accurate calculation of eye diagrams and bit error rates in opticaltransmission systems using linearization

We present a novel linearization method to calculate accurate eye diagrams and bit error rates (BERs) for arbitrary optical transmission systems and apply it to a dispersion-managed soliton (DMS) system. In this approach, we calculate the full nonlinear evolution using Monte Carlo methods. However, we analyze the data at the receiver assuming that the nonlinear interaction of the noise with itself in an appropriate basis set is negligible during transmission. Noise-noise beating due to the quadratic nonlinearity in the receiver is kept. We apply this approach to a highly nonlinear DMS system, which is a stringent test of our approach. In this case, we cannot simply use a Fourier basis to linearize, but we must first separate the phase and timing jitters. Once that is done, the remaining Fourier amplitudes of the noise obey a multivariate Gaussian distribution, the timing jitter is Gaussian distributed, and the phase jitter obeys a Jacobi-Θ distribution, which is the periodic analogue of a Gaussian distribution. We have carefully validated the linearization assumption through extensive Monte Carlo simulations. Once the effect of timing jitter is restored at the receiver, we calculate complete eye diagrams and the probability density functions for the marks and spaces. This new method is far more accurate than the currently accepted approach of simply fitting Gaussian curves to the distributions of the marks and spaces. In addition, we present a deterministic solution alternative to the Monte Carlo method     

Inverter harmonic reduction using Walsh function harmonicelimination method

A pulse-width-modulated (PWM) inverter using the Walsh function harmonic elimination method is proposed in this paper. By using the Walsh domain waveform analytic technique, the harmonic amplitudes of the inverter output voltage can be expressed as functions of switching angles. Thus, the switching angles are optimized by solving linear algebraic equations instead of solving nonlinear transcendental equations. The local piecewise linear relations between the switching angles and the fundamental amplitude can be obtained under an appropriate initial condition. By searching all feasible initial conditions, the global solutions are obtained. The relations between switching angles and fundamental amplitude can be approximated by straight-line curve fitting. Thus, on-line control of fundamental amplitude and frequency is possible for the microcomputer-based implementation. The developed algorithm can be applied to both bipolar and unipolar switching schemes. The theoretical predictions are confirmed by computer simulations and DSP-based hardware implementation     

V-band harmonic injection-locked frequency divider using cross-coupled FETs

A V-band 1/2 frequency divider is developed using harmonic injection-locked oscillator. The cross-coupled field effect transistors (FETs) and low quality-factor microstrip resonator are employed as a wide-band oscillator to extend the locking bandwidth. The second harmonic of free-running oscillation signal is injected to the gates of cross-coupled FETs for high-sensitivity superharmonic injection locking. The fabricated microwave monolithic integrated circuit frequency divider using 0.15-/spl mu/m GaAs pHEMT process showed a maximum locking range of 7.4 GHz (from 65.1 to 72.5 GHz) under a low power dissipation of 100 mW. The maximum single-ended output power was as high as -3 dBm.     

High-performance induction motor speed control using exact feedback linearization with state and state derivative feedback

This paper presents a novel nonlinear speed/position control strategy for the induction motor utilizing exact feedback linearization with state and state derivative feedback. The speed/position and flux control loops utilize nonlinear feedback which eliminates the need for tuning, while ordinary proportional-integral controllers are used to control the stator currents. The control scheme is derived in rotor field coordinates and employs an appropriate estimator for the estimation of the rotor flux angle, flux magnitude, and their derivatives. The overall control scheme can be easily implemented with a microprocessor-based control platform. An error sensitivity analysis is included which proves the system to be robust to parameter variation and even more, immune to rotor resistance variation. Simulation and experimental results validate the theoretical part of the paper and reveal the high performance and advantages of the novel control scheme.     

New percentage linearization measures of the degree of linearization of HPA nonlinearity

A percentage linearization (PL) and a percentage linearization area (PLA) are defined and proposed as two new measures of the degree of AM-to-AM linearization, achieved by a linearization scheme, of the saturating region of nonlinear high power amplifiers (HPAs). The measures are based on defining a HPA saturation power point which is very close to the asymptotic output saturation power (AOSP) line. Here the point where the first derivative of the AM-to-AM dB power characteristic drops to 0.1 is so defined. Employing the passage through a typical pseudomorphic high electron mobility transistor solid-state power amplifier (PHEMT SSPA) of an UWC-136 8-PSK signal, as a test signal, an introductory insight into the usefulness of these linearization measures is presented.